1. Field of the Invention
This invention relates to cathode electrode structures for sodium sulphur cells.
2. Prior Art
In a sodium sulphur cell, a solid electrolyte material separates molten sodium, forming the anode, from a sulphur/polysulphide cathodic reactant. The solid electrolyte is a material, such as beta-alumina, which conducts sodium ions. On discharge of the cell, the sodium gives up electrons at the anodic interface of the solid electrolyte. Sodium ions pass through the electrolyte into the cathode adjacent the opposite face of the electrolyte. The electrons pass through the sodium to the anode current collector and thence around an external circuit to a cathode current collector, e.g. a rod or tube formed of or coated with a material chemically inert to the cathodic reactant. The electrons must pass from this cathode current collector to the region of the cathode adjacent the surface of the solid electrolyte where they react with the sulphur to form sulphide ions. Sulphide ions and sodium ions form a polysulphide. The electronic conductivity of molten sulphur is low and hence it is the practice to pack the cathodic regions with a fibrous carbon or graphite material to provide the required electronic conductivity, the fibrous material forming a matrix through which the cathodic reactant can move.
Sodium sulphur cells are commonly of tubular form. They may of the kind known as a central sodium cell in which the sodium is inside the electrolyte tube and the cathodic region lies between the outer surface of the electrolyte tube and a tubular current collector which might constitute or form part of the cell housing. Alternatively, the cell may be of the type known as a central sulphur cell in which the sodium is outside the electrolyte tube and the cathodic reactant is in an annular region between the inner surface of the electrolyte tube and a central current collector rod or tube. In each of these constructions, the cathodic region is of annular form. The common practice has been to use graphite felt as the electronically-conductive packing material in the cathodic region. Such felt may be formed, for example, into annular elements which may be packed axially into the cathodic region, the felt subsequently being impregnated with sulphur.
The matrix material in the cathodic region has to be porous to permit free access of the cathodic reactant material to the neighbourhood of the electrolyte. Electrically however this conductor forms the path to transfer electrons from the reaction zone to the cathode current collector when charging the cell and provides the path between the current collector and the regions near the surface of the electrolyte where the sulphide ions have to be formed on discharge of the cell.
One of the problems in sodium sulphur cells is to obtain sufficient overall conductance in the cathodic region. The conductance of the carbon matrix material in the cathodic regions constitutes one of the limitations on the performance of such cells. It is possible to increase the bulk conductance of graphite felt by packing the felt more tightly. This however impedes the free movement of the cathodic reactant material which must have access to the neighbourhood of the electrolyte. Other techniques have therefore been proposed to improve the bulk conductance in the cathodic region. For example in U.S. Pat. No. 4,052,535 there is described a cathode matrix for a sodium sulphur cell formed of a plurality of discrete elements with electronically-conductive material such as graphite foil between the elements and extending across the region between the current collector and the electrolyte to increase the conductivity across that region.
It is also known to employ loose fibres instead of felt in the cathodic region of a sodium sulphur cell, as described in U.S. Pat. No. 4,118,545. The fibre material may be packed between layers of cloth and U.S. Pat. No. 4,118,545 discloses the utilisation of the cloth to join a plurality of elongate elements, along their length, so that the assembly can be formed into an annular unit to fit within a cell. This assembly has to be impregnated with sulphur after it has been formed. That specification more particularly describes the use of a mixture of graphite or carbon fibres with fibres of another material, for example an oxide material such as alumina or zirconia, which is preferentially wetted by the sulphides in the cathodic reactant, to improve the physical transfer of the cathodic reactant material.
In U.S. Pat. No. 4,076,902 there is described a sodium sulphur cell in which graphite fibres are arranged in an annular region between a beta alumina electrolyte tube and a surrounding cell housing with the fibres in a direction normal to the cathodic current collector constituted by the cell housing and to the electrolyte tube.
In the aforementioned co-pending application, I have described a method of making an annular cathodic electrode, for a sodium sulphur cell, the electrode having a porous matrix of electronically conductive material impregnated with an electro-chemical reactant material, which method comprises the steps of shaping a sheet of compressible matrix material, and impregnating it, either before or after the shaping, with the reactant material, the shaping and impregnating being effected at a temperature above the melting point of the reactant material, cooling the impregnated shape to solidify the reactant, the shaping operation compressing the matrix material over its whole area and effecting greater compression of the material in preselected regions so that the matrix material is formed into a planar assembly of shaped segments, which are each a portion of an annulus whereby the shaped segments are formable into the required annular structure. If the compressible material is sheet material with the fibres lying generally parallel to the plane of the sheet, this results in the formation of an electrode with fibres extending parallel to the surfaces of the electrolyte and current collector.